Safety control system for fuel burners



Dec. 139 1949 P. R. MARVIN 2,491,125

SAFETY CONTROL SYSTEM FOR FUEL BURNERS Filed Feb.. 15. 1947 f fm/gm,-

P/zzo E www@ Patented Dee. 13, 1949 2,491,125 SAFETY coN'rnor. SYSTEMFon FUEL mmm-ms Philip R. Marvin, Wauwttosa, Wis., assigner 'toMilwaukee Gas Specialty Company, Milwaukee,

^ l, Wis., a corporation o! Wisconsin Application February l5, 1947,Serial No.'728,'893

Claims. (BLISS-1173.)

This invention relates, in general, to a burner control sysem, and hasparticular relation to burner control systems employing means .forinterrupting the supply of fuel to a main burnerln the event a pilotburner is extinguished.

While the particular system which I shall d'escribe hereinafter inconnection with the drawing is, in general, of the type in which anelectric circuit for an electroresponsive valve in the fuel conduit hascontacts under the control of a relay in the controi circuit, it 1ste beunderstood that the invention in its broader aspects is not limited tothis particular type of system but may be embodied in other types ofsystems as suitable or desired;

Prior systems of the type with which the present invention is concerned-have proposed the use in position, for example, where it is subject tothe heat of a pilot burner, of a thermistor which has a negativetemperature coeiilcient of resistivity and operates, when heated by thepilot burner, to pass suilicient current to maintain anelectroresponsive valve -in the fuel conduit open when the pilot burneris burning and the circuit including the electroresponsive valve isotherwise closed, and to reduce the current so that theelectroresponsive valve operates to Aclosed position when the pilotiiame is extinguished.

` In these prior schemes however, the thermistor has usually'beensubjected, for example, to the heat within the burner compartment and/orto the heat 0f the main burner or other ambient temperature. As aresult, prompt actuation of the valve to closed positionv when the pilotiiame is extinguished is not assured because the ambient temperaturemayv heat the thermistor an amount to pass suiiicient current tomaintain the electroresponsive valve in open position for a considerableperiod of time after the pilot flame is extinguished.

With the valve in the fuel condit open and the pilot ilame extinguished,fuel may ilow in considerable volume 'to the main burner. As a re'- sultthe burner compartment may be filled, -foiexample, with a highlyexplosive gas-air mixture, thus presenting the possibility of seriousAand disastrous explosion when it is attempted to relight the pilotburner. These things can happen and have happened in using defectiveso-called safety devices, and, profiting by experience, 'therequirements of the American Gas Association that such-devices be safehave become increasingly stringent.

One of the main objects of the present invention is to provide animproved burner control system in which the problems previouslypresented are overcome.

Another object of 'theinvention is to'provide a 'burner control systeminwhich prompt actuation of the valve in the fuel conduit to closedposition "when the pilot, name is -extinguished is assured, and, moreparticularly, a system in which the valve cannot be maintained open orits closing retarded, for example, by heat within the burner compartmentand/or the-heat of the main burner or other ambient temperature.

Another object of the invention is to provide I -a burner control system'using thermistor type and contacts ln'the first circuit controlled byenergization of the coil, a pilot burner, a rst thermistor -having -anegative coeiiicient of resistivity connected in'series with'the -relaycoil and positioned to be subjected to the heat of the pilot ilame so asto pass 'suilicient current through the coil to cause the valve to beopened and maintained open when the pilot burner is lighted, saidthermistor reducing 'the current through said relay coil to causeclosing of the valve when the pilot name is extinguished, a shuntcircuit connected to the second circuit around the relay coil, and asecond thermistorfhaving a negative coleilicient of resistivitypositioned to be subjected to ambient temperature and connected into-th'e Ashunt circuit in series with the relay coil with the circuits forthe first and second thermistors so calibrated that, when the system isin proper operation, the holding current for the relay is reduced tosuch value that the 'relay is just maintained energized to close thecontacts in the circuit for the valve due to shunting fof currentthrough the -shunt circuit including the second thermistor. y

Another object of 'the invention is lto provide a system vas set -forthinthe `preceding paragraph wherein there is a iirst resistor in thesecond circuit in series with the relay coil and the iirst thermistorand a second resistor in the shunt circuit in series with the r'elaycoil and the second thermistor for calibrati'ng and adjusting thesystem.

shown at I is a main burner.

its simplicity in construction, its economy in manufacture, itseifectiveness in use, and the absence of moving contacts and the like inthe devices subjected to the heat of the pilot flame and to the ambienttemperature.

Further objects and advantages of the invention will appear from thefollowing detailed description, taken in connection with theaccompanying drawing in which thesingle gure is a diagram showingschematically the parts and circuit connections of one form of systemembodying the present invention.

Referring now to the drawing, the burner It may be the main burner for aroom or space heater, or it may be the burner for a water heater or fora iloor furnace, an over burner, one or more top burners for a gasrange, or any other burner, and of any suitable construction.

A fuel supply pipe or conduit 2 leads to the burner i for the deliveryof gaseous or other fuel thereto, for example, through a mixing chamber3 to which air is admitted through adjustable aid inlets (not shown), aswell understood in the art. The flow of gas through the conduit 2 iscontrolled by a valve 4 of any suitable electroresponsive orelectrically operated type. For purposes of illustration, the valve 4may be a solenoid valve operated to open position by the iiow of currentthrough its coil Band to closed position by a spring 1.

The pilot burner 8, which is located in juxtaposition to the main burnerI to maintain a pilot flame 9 for igniting the main burner, is suppliedwith gaseous fuel by a pilot supply pipe IIl. The pilot supply pipev Imay be connected, for example, to the pipe 2 anterior of the valve 4, orthrough this valve, or in some cases posterior of the valve, or to anyother source of fuel supply.

Power for operating the valve 4 is supplied from a suitable source, forexample the line wiresA I2 of a source of alternating current such asthe type of alternating current linewhich averages about 115 volts. Thecircuit through the coil i of the valve 4 is controlled, for example, bya relay I3 having a coil I4 and a pair of contacts I5. One contact I5 isconnected in circuit with one of the line wires I2. The other contact I5is connected by a conductor I6 to one side of the coil 6, and the otherside of the coil 6 is connected in circuit with the other line wire I2.

A thermistor I8 is positioned so as to be heated by the pilot ilame I9when the pilot llame is burning. The thermistor I 3, which is preferablynot positioned dire/ctly in the llame, has a negative temperaturecoeiilcient of resistivity, i. e., it is a non-conductor, or relativelya non-conductor, when not subjected to the heat of the pilot name or acorresponding temperature, and changes to an electric conductor whenheated by the pilot flame or a corresponding temperature.

One side of the thermistor I3 is connected by a conductor 2l to one side2I of a suitable voltage source. A conductor 22 connects the other sideof the thermistor I8 through an adjustable resistor 23 and the coil I4of the relay I3 to the other side 24 of the voltage source. 'I'he coilI4 of relayI3. adjustable resistor 23, and thermistor I8 are thusconnected in series in an electric circuit leading, for example, fromone side of the voltage source through the thermistor, adjustableresistor, and relay coil to the other side 24 of the voltage source.

'Ihe adjustable resistor 23 is preferably adjusted to pass just enoughcurrent to cause the the thermistor I8 is heated by the pilot flame andthere is suilicient diierential in the pilot llame temperature and theambient temperature, as will presently appear.

A second tlf.` ermistor 26 is connected in a shunt circuit around thecoil I4 of the relay I3, and the shunt circuit includes a resistor 2'Iin series with the Athermistor 26. This second thermistor 26 has anegative temperature coeflicient of resistivity. The shunt circuitcomprises a conductor 28 connecting one side of the thermistor 26 to theconductor 2I to one side of relay coil I4, and a conductor 29 connectingthe other side of the thermistor 26 to the conductor 22 on the otherside of the relay coil I4.

The thermistor 26 is positioned so as to be subjected to the ambienttemperature which may be the heat within the compartment (not shown) forthe main burner and/or the heat of the main burner or other ambienttemperature. As previously stated, the adjustable resistor 23 ispreferably adjusted to pass just enough current to cause the relay I3 tomaintain the contacts I5 closed and thereby to maintain the valve 4 openas long as the thermistor is heated by the pilot flame and there issufficient differential in the pilot name temperature and the ambienttemperature. The desired balance can be obtained by connecting suitablemilliameters in the circuits for the respective thermistors I8 and 23and noting the readings in adjusting the resistor 23.

In operation, with the heat' of the pilot flame I9 heating thethermistor I8 and the heat of the ambient temperature on the thermistorIl suihciently lower than the heat to which this thermistor is subjectedby the pilot ilame the thermistor I8 passes current through the relaycoil I4 suilicient to close contacts I5. 'I'he voltage across the linewires I2 is thus applied to the coil 6 of the valve 4, opening thisvalve and holding it open for the supply of fuel to the burner I as longas this condition exists.

Ihe resistor 23 constitutes an adjustable protective device for thethermistor I8, and the resistor 21 constitutes a suitable protectivedevice for the thermistor 26. The resistors constitute current limitingdevices for preventing destruc- 50 tive overheating of the thermistors.

If during operation of the system with the thermistor I8 heated by thepilot flame sumciently to maintain the valve 4 open and the ambienttemperature is sinsuillcient to heat the 55 thermistor 26 sufilcientlyto by-passfthe current around the relay I3 so as to cause closing of thevalve 4, and the pilot flame is extinguished, the following actionoccurs. The thermistor I8 immediately cools and becomes non-conductingor 6o sumciently non-conducting so that it will not pass sumcientcurrent to cause relay I3 to hold contacts I5 closed. These contactsthen open the circuit for the valve 4, which operates to closed positionshutting oif the supply of fuel to 65 the burner I.

To close the circuit which opens the valve 4 and maintains it open, thepilot burner is lighted and the ilame I3 issuing therefrom heats thethermistor I8 and causes it to pass suillcient cur- 70 rent through therelay I3 to close contacts II. This energizes the coil 6 of the valve 4,which thereby operates to open position to pass fuel to the main burnerI. It is thus'apparent that no passage of gas to the burner I can occurun- 75 less the pilot flame is burning and the ambient It is to beunderstood that the electric circuit for controllingthe valve l may havea thermostat, limit device, or other control means therein, in whichcase the .valve 4 will be caused to open and to be held open by the heatof the pilot iiame on the thermistor I8 only when such thermostat, limitdevice, or other control is in position closing the circuit whichincludes the thermistor I8 and the, thermistor 26 is not heated by theambient temperature suillciently to by-pass the coil I4 of the relay I3.

The thermistors I8 and 26 may be of any suitable form. Such devices arewell known and will not be described in detail. Sumce it to state thatthe thermistor elements may be formed, for example, of ierric oxide(FezOs) with about 1% of titanium dioxide (T102), of suitable particlesize, or by mixing oxides with an organic binder and solvent andapplying suitable contacts, or otherwise as desired.

The embodiment of the invention shown in the drawing is for illustrativepurposes on1y, and it is to be expressly understood that said drawingand the accompanying speciiication are not to be construed as adefinition of the limits or scope of vthe invention. reference being hadto the appended claims for that purpose.

I claim:

1. In a system of the class described, in combination, a fuel conduit,an electroresponsive valve in said conduit. a iirstelectric circuit forsaid valve, a second electric circuit. a relay having a coil in saidsecond circuit and contacts in said iirst circuit controlled byenergization of said coil, a pilot burner, a first thermistor having anegative coeilicient of resistivity connected in series with said relaycoil and positioned to be subjected to the heat of the pilot iiame so asto pass suiilcient current through said coil to cause said valve to beopened and maintained open when the pilot burner is lighted, saidthermistor reducing the current through said relay coil to cause closingof said valve when the pilot iiame is extinguished, a shunt circuitconnected to said second circuit around said relay coil, and a secondthermistor having a negative coemcient of restivity positioned to besubjected to ambient temperature and connected into said shunt circuitin series with said relay coil. the circuits /for said ilrst and secondthermistors being so calibrated that when the systeml is in properoperation the holding current for the relay is reduced to such valuethat the relay is just maintained energized to close the contacts in thecircuit for the electroresponsive valve due to shunting of the currentthrough the shunt circuit including said second thermistor.

2. A system according to claim 1 wherein there is a iirst resistor insaid second circuit in series with said relay coil and said rstthermistor, and

a second resistor in said shunt circuit in series with said relay .coiland said second thermistor for calibrating and adjusting the system.

3. A system according to claim 1 wherein there is a main burner injuxtaposition to said pilot burner and wherein said second thermistor-is ing a negative coeiicient of resistivity connected in series withsaid relay coil and positioned to be subjected to the heat of a dame soas to pass sumcient current through said coil to cause saidelectroresponsive device to be operated when the llame is burning, saidthermistor reducing the current through said relay coil to causeoperation of the electroresponsive device to inoperative position whenthe iiam'e is extinguished, a shunt circuit connected to said secondcircuit around said relaycoil, and a second thermistor having a negativecoeilicient of resistivity positioned to be subject to ambienttemperature and connected into said shunt circuit in series with saidrelay coil, the circuit for said first and second thermilstors being socalibrated that when the system is in proper operation the holdingcurrent for the relay is reduced to such value that the relay is Justmaintained energized to close the contacts in the circuit for theelectro-responsive device due to shunting through th shunt circuitincluding said second thermistor.'

5. A system according to claim 4 wherein there is a first resistor insaid second circuit inseries with said relay coil and said iirstthermistor, and a second resistor in said shunt circuit in series withsaid relay coil and said second -thermistor for ealibrating andadjusting the system.

PHILIP R. MARVIN.

REFERENCES crrED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Certificate of Correction Patent No. 2,491,125 lDecember 13, 1949 PHILIP R. MARVIN It is hereby certified that errorsappear in the printed specification of the above numberedpatent-,requiring correction as follows:

Column 5, line 49, for restivity read resistivity; column 6, line 3,6,for th" read the; i l

land that the said Letters Patent should u,be read with thesecorrections therein that the same may conform to the record of the casin the Patent Office.

Signed-and sealed this 11th day of April, D. 1950.

s im] THOMAS F. MURPHY,

Assistant Uommzssz'oner of Patents.

